Summary
Highlights
Electric motors are found in many everyday appliances like toy cars, fans, washing machines, and blenders. The fundamental principle behind an electric motor is that a current-carrying conductor placed in a magnetic field will experience a force. This video will explore this principle and how it's used to design electric motors.
A current-carrying wire produces a magnetic field. Consequently, a current-carrying wire exerts a force on a magnet, and conversely, a magnet exerts a force on a current-carrying wire. Michael Faraday observed in 1821 that when current passes through a wire in a magnetic field, a force is exerted on the wire, causing it to move. This force is maximum when the conductor is perpendicular to the magnetic field and zero when it's parallel.
The magnitude of the force (F) on a current-carrying conductor placed perpendicular to a magnetic field depends on three factors: the strength of the magnetic field (B), the current (I) flowing in the wire, and the length (L) of the wire within the magnetic field. The formula for this force is F = BIL (in SI units), where B is measured in Tesla (T) or Weber per meter squared.
Fleming's left-hand rule helps determine the direction of the force on a current-carrying conductor in a magnetic field. By holding the forefinger, center finger, and thumb of the left hand at right angles, the forefinger indicates the magnetic field direction (North to South), the center finger indicates the conventional current direction, and the thumb indicates the direction of the force on the conductor.
Electric motors convert electrical energy into mechanical (rotational kinetic) energy. This video focuses on the DC motor, which uses direct current (current flowing in one direction) from sources like a cell. While a single straight wire experiences linear motion, rotational motion in an electric motor is achieved by applying two equal and opposite forces, known as a couple, usually on a rectangular coil.
A DC motor consists of a magnetic field (North to South poles), a rectangular coil (armature coil ABCD) made of insulated copper wire, a commutator (split rings) to ensure current flows in the right direction, a shaft on which the coil and commutator rotate, a battery as the voltage supply, and carbon brushes to maintain electrical contact with the rotating commutator without twisting wires.
When current flows through the rectangular coil within the magnetic field, forces act on the sides of the coil. Using Fleming's left-hand rule, side AB experiences an upward force, and side CD experiences a downward force. Sides BC and AD, being parallel to the magnetic field, experience no force. These opposing forces create a couple, causing the coil to rotate clockwise. The commutator reverses the current direction every half rotation, ensuring continuous rotation in the same direction.
To increase the rotational speed of the motor, the force on the coil needs to be increased. This can be achieved by: 1) Increasing the strength of the magnetic field (e.g., by inserting a soft iron core), 2) Increasing the current in the coil, 3) Increasing the length of the wire (by increasing the area of the coil), and 4) Increasing the number of turns in the coil.